The present invention relates to the preparation of single crystal semiconductor wafers and more particularly to utilizing and producing a polycrystalline CVD diamond substrate therefor.
Its hardness and thermal properties are but two of the characteristics that make diamond useful in a variety of industrial components. Initially, natural diamond was used in a variety of abrasive applications. With the ability to synthesize diamond by high pressure/high temperature (HP/HT) techniques utilizing a catalyst/sintering aid under conditions where diamond is the thermally stable carbon phase, a variety of additional products found favor in the marketplace. Polycrystalline diamond compacts, often supported on a tungsten carbide support in cylindrical or annular form, extended the product line for diamond additionally. However, the requirement of high pressure and high temperature has been a limitation in product configuration, for example.
Recently, industrial efforted directed toward the growth of diamond at low pressures, where it is metastable, has increased dramatically. Although the ability to produce diamond by low-pressure synthesis techniques has been known for decades, drawbacks including extremely low growth rates prevented wide commercial acceptance. Recent developments have led to higher growth rates, thus spurring recent industrial interest in the field. Additionally, the discovery of an entirely new class of solids, known as "diamond like" carbons and hydrocarbons, is an outgrowthh of such recent work.
Low pressure growth of diamond has been dubbed "chemical vapor deposition" or "CVD" in the field. Two predominant CVD techniques have found favor in the literature. One of these techniques involves the use of a dilute mixture of hydrocarbon gas (typically methane) and hydrogen wherein the hydrocarbon content usually is varied from about 0.1% to 2.5% of the total volumetric flow. The gas is introduced via a quartz tube located just above a hot tungsten filament which is electrically heated to a temperature ranging from between about 1750.degree. to 2400.degree. C. The gas mixture disassociates at the filament surface and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate is held in a resistance heated boat (often molybdenum) and heated to a temperature in the region of about 500.degree. to 1100.degree. C.
The second technique involves the imposition of a plasma discharge to the foregoing filament process. The plasma discharge serves to increase the nucleation density, growth rate, and it is believed to enhance formation of diamonds films as opposed to discrete diamond particles. Of the plasma systems that have been utilized in this area, there are three basic systems. One is a microwave plasma system, the second is an RF (inductively or capacitively coupled) plasma system, and the third is a d.c. plasma system. The RF and microwave plasma systems utilize relatively complex and expensive equipment which usually requires complex tuning or matching network to electrically couple electrical energy to the generated plasma. Additionally, the diamond growth rate offered by these two systems can be quite modest.
In the semiconductor arena, doping polycrystalline CVD diamond with boron, aluminum, lithium, phosphorous, or the like to produce an n-type or p-type semiconductor device is known. Representative art in this regard include U.S. Pats. Nos. 3,636,077, 3,636,078, 3,636,079, and 4,767,608, and European Patent Publication No. 286,306. EP Patent Publication 282,054 expands on such semiconductor technology by coating a single crystal substrate of silicon or GaAs with an intermediate SiC layer upon which is deposited CVD diamond as a single crystal. This single crystal diamond can be doped with boron, phosphorous, or sulfur, for example. This publication reports that directly growing CVD diamond on a single crystal silicon substrate results in the production of polycrystalline CVD diamond, rather than single crystal diamond, and hence is a process not suitable for semiconductor production.